Process for preparing R-(-)-carnitin from S-(-)-chlorosuccinic acid or from a derivative thereof

ABSTRACT

An inner salt of L-carnitine is prepared by reduction, with a suitable reducing agent, of a compound of formula (I):  
                 
 
     where X 1  and X 2 , which may be the same or different, are hydroxy, C 1 -C 4  alkoxy, phenoxy, halogen, or X 1  and X 2 , when taken together are an oxygen atom and the resulting compound is a derivative of succinic anhydride; Y is halogen, a mesyloxy or a tosyloxy group, and subsequent treatment with water, then with a base and then with trimethylamine.

[0001] The invention described herein relates to a process for thepreparation of R-(−)-carnitine (L-(−)-carnitine orR-(−)-3-hydroxy-4-(trimethylammonium)butyrate), hereinafter referred to,for the sake of brevity, as L-carnitine, starting fromS-(−)-chlorosuccinic acid or one of its derivatives.

BACKGROUND TO THE INVENTION

[0002] As is known, carnitine possesses an asymmetrical carbon atom andthe enantiomer L-carnitine is the isomer present in living organisms,where it is essential for fatty acid metabolism and functions activelyin the transport of fatty acids across the mitochondrial membranes. Forthis reason L-carnitine, in addition to being a life-saving drug forthose who suffer from an L-carnitine deficiency of genetic origin and tobeing used in cases of temporary L-carnitine deficiency, such as, forinstance, those occurring after haemodialysis (U.S. Pat. No. 4,272,549,Sigma-Tau), plays an important role in energy metabolism and is regardedas a non-toxic natural product capable of enhancing cardiac function. Itis therefore used as a support drug in the treatment of various heartdiseases such as ischaemia, angina pectoris, arrhythmias, etc. (U.S.Pat. No. 4,649,159 and U.S. Pat. No. 4,656,191 Sigma-Tau). L-carnitineand its derivatives, moreover, have also been used to a significantextent as serum lipid lowering agents, anticonvulsants and blood productpreservatives. Recently, one of its derivatives, propionyl L-carnitine(Dromos®), was launched on the Italian market for the treatment ofintermittent claudication (U.S. Pat. No. 4,968,719, EP 0793962,Sigma-Tau).

[0003] There is also a substantially growing use of L-carnitine as afood supplement in the field of the so-called ‘health foods’ or“nutraceuticals”.

[0004] All this explains why L-carnitine is produced industrially inlarge amounts and also why several attempts have been made to improvethe industrial synthesis of L-carnitine in terms of the cost of theproduct.

[0005] From a general point of view, the synthesis pathways that can beused to synthesise L-carnitine are essentially three.

[0006] The first of these, starting from non-chiral or racemiccompounds, passes through racemic intermediates, at the level of one ofwhich the separation of the useful enantiomer occurs, with methods knownto experts in pharmaceutical technology. Though this synthesis pathwaypresents the advantage of being able to rely on starting materials witha relatively low cost, for example, racemic carnitinamide (U.S. Pat. No.4,254,053, Sigma-Tau); racemic 2,3-dichloro-1-propanol (N. Kasai and K.Sakaguchi, Tetrahedron Lett. 1992, 33, 1211); 3-butenoic acid (D.Bianchi, W. Cabri, P. Cesti, F. Francalanci, M. Ricci, J. Org. Chem.,1988, 53, 104); racemic 3-chloro-2-hydroxy-trimethylammonium chloride(R. Voeffray, J. C. Perlberger, L. Tenud and J. Gosteli, Helv. Chim.Acta, 1987, 70, 2058); racemic epichlorohydrin (H. Loster and D. M.Muller, Wiss. Z. Karl-Marx-Univ. Leipzig Math.-Naturwiss. R. 1985, 34,212); diketene (L. Tenud, Lonza, DE 2,542,196, 2,542,227 and DE2,518,813), it also presents a serious drawback, in that, at the momentone wishes to isolate the useful enantiomer from a racemic mixture,there is a theoretical loss of at least 50% of the product on which saidseparation is operated. In practice, then, the yields in this synthesisstep are substantially lower (U.S. Pat. No. 4,254,053, Sigma-Tau) andthere is the drawback of having to recover the chiral compound used forthe separation of the racemic mixture.

[0007] The second synthesis pathway, again starting from non-chiralproducts, “creates” the chiral centre of the configuration desired,operating a synthesis step in a chiral environment, whether by means ofa catalyst (H. C. Kolb, Y. L. Bennani and K. B. Sharpless, Tetrahedron:Asymmetry, 1993, 4, 133; H. Takeda, S. Hosokawa, M. Aburatani and K.Achiwa, Synlett, 1991, 193; M. Kitamura, T. Ohkuma, H. Takaya and R.Noyori, Tetrahedron Lett., 1988, 29, 1555), or by means of an enzyme(U.S. Pat. No. 4,707,936, Lonza). The disadvantages of this pathway arethe high cost of the catalysts and the fact that, at the time the chiralcentre is created catalytically, one is normally unable to obtain thepure enantiomer, but mixtures are obtained with variable enantiomericexcesses of the useful isomer, with all the consequent difficulties ofhaving to separate two substances with the same physico-chemicalcharacteristics. In the case of the use of micro-organisms incontinuous-cycle reactors, the transformation of the starting productsinto end products is never complete and the end product has to bescrupulously purified of all organic impurities of cellular origin,which are dangerous in that they are potential allergens.

[0008] The third synthesis pathway involves the use of a chiral startingproduct, which is transformed into L-carnitine via a series of reactionswhich, if the chiral centre is affected, must be stereospecific, whichmeans that the stereochemistry of said centre must be maintained orcompletely inverted during the reaction, which is not always easy toachieve. If, on the other hand, the synthesis step does not affect thechiral centre, the enantiomeric excess (ee) of the end product must bethe same, or very close to, the starting product, which means that“racemising” reaction conditions must be carefully avoided. Anotherlimitation is the cost of the chiral starting products, which isnormally much higher than that of non-chiral products. The effect ofthese difficulties has been that none of the various processes startingfrom chiral products such as, for example, la R-(−)-epichlorohydrin (M.M. Kabat, A. R. Daniewski and W. Burger, Tetrahedron: Asymmetry, 1997,8, 2663); D-galactono-1,4-lactone (M. Bols, 1. Lundt and C. Pedersen,Tetrahedron, 1992, 48, 319); R-(−)-malic acid (F. B. Bellamy, M.Bondoux, P. Dodey, Tetrahedron Lett. 1990, 31, 7323);R-((+))-4-chloro-3-hydroxybutyric acid (C. H. Wong, D. G. Drueckhammerand N. M. Sweers, J. Am. Chem. Soc., 1985, 107, 4028; D. Seebach, F.Giovannini and B. Lamatsch, Helv. Chim. Acta, 1985, 68, 958; E.Santaniello, R. Casati and F. Milani, J. Chem. Res., Synop., 1984, 132;B. Zhou, A. S. Gopalan, F. V. Middlesworth, W. R. Shieh and C. H. Sih;J. Am. Chem. Soc., 1983, 105, 5925); 4-hydroxy-L-proline (P. Renaud andD. Seebach, Synthesis, 1986, 424); (−)-p-pinene (R. Pellegata, I. Dosi,M. Villa, G. Lesma and G. Palmisano, Tetrahedron, 1985, 41, 5607);L-ascorbic acid or arabinose (K. Bock, I. Lundt and C. Pederson; ActaChem. Scand., Ser. B, 1983, 37, 341); D-mannitol (M. Fiorini and C.Valentini, Anic, EP 60.595), has to date been used for the industrialproduction of L-carnitine.

[0009] A case apart is the Sigma Tau Italian patent No. 1,256,705, whichmay be regarded as a mixture of the first and second synthesis pathways.What it describes, in fact, is the preparation of L-carnitine startingfrom D-((+))-carnitine, obtained as a discard product from theL-carnitine preparation process by resolution of the carnitinamideracemic mixture by means of camphoric acid (U.S. Pat. No. 4,254,053,Sigma-Tau).

[0010] The bibliographical and patent references cited above merely givesome idea of the vast body of work carried out in order to find aneconomically advantageous synthesis of L-carnitine. The fact is that theonly two processes which have proved industrially and economically validare those used by the two main manufacturers of L-carnitine, Sigma-Tauand Lonza, as described in the two above-mentioned patents, U.S. Pat.No. 4,254,053 and U.S. Pat. No. 4,708,936, which date back to 1978 and1987, respectively.

SUMMARY OF THE INVENTION

[0011] A process has now been found which starts from a chiral productand solves all the problems of the “third pathway”, that is to say theproblem of the cost of the starting product and those of thestereospecificity and regiospecificity of the reactions necessary inorder to pass from S-(−)-chlorosuccminc acid, or one of its derivatives,to L-carnitine. The L-carnitine obtained is, in fact, particularly pure,with a D-carnitine percentage ≦0.2%.

[0012] In particular, the invention described herein relates to aprocess for the preparation of L-carnitine inner salt which includes thereduction, with a suitable reducing agent, of a compound of formula (I)

[0013] where:

[0014] X₁ and X₂, which may be the same or different, are hydroxy, C₁-C₄alkoxy, phenoxy, halogen; or X₁ and X₂, when taken together are anoxygen atom and the resulting compound is a derivative of succinicanhydride;

[0015] Y is halogen, the mesyloxy or the tosyloxy group;

[0016] and subsequent treatment with a base and then withtrimethylamine.

[0017] Examples of C₁-C₄ alkoxy groups are methoxy, ethoxy, propoxy,isopropoxy, butoxy, isobutoxy and ter-butoxy. The methoxy and ethoxygroups are preferred. Examples of halogen are chlorine, bromine andiodine. Chlorine is preferred.

[0018] The reduction of the compound of formula (I) is done with asuitable reducing agent, which may be selected from those available bythose having ordinary experience in the field on the basis of their owngeneral knowledge of the sector. Reducing agents suitable forimplementing the process according to the invention described herein arehydrides. Examples of hydrides are diborane, mixed hydrides such aslithium and aluminimum hydride, lithium or sodium borohydride. Thechoice of a suitable reducing agent will be made in relation to thecompound of formula (I) to be treated. This choice is made by the personhaving ordinary experience in the field on the basis of his or hergeneral knowledge and no further explanation is necessary.

[0019] The process according to the invention is carried out in asuitable reaction medium, such as an organic solvent, preferablyaprotic, for example, tetrahydrofuran (THF), dioxane, ethylene glycoldimethyl ether (DME) or 2-methoxyethyl ether (Diglime).

[0020] The reaction temperature, reactant concentrations and all otherparameters useful for determining the reaction conditions can beobtained by consulting normal organic chemistry manuals.

[0021] In a first embodiment of the invention, the compound of formula(I) is S-(−)-chlorosuccinic acid (X₁ and X₂ are hydroxy and Y ischlorine). Said acid can be prepared with good yields and sterospecificreaction, e.g. from L-aspartic acid (S-((+))-aspartic acid) (J. A.Frick, J. B. Klassen, A. Bathe, J. M. Abramson and H. Rapoport,Synthesis, 1992, 7, 621 and literature cited therein), or can bepurchased on the market.

[0022] In this first embodiment, the reducing agent is diborane.

[0023] Carnitine inner salt is then obtained from the reduction productof S-(−)-chlorosuccinic acid, without the isolation of any intermediateproduct, by treatment with aqueous sodium hydroxide and trimethylamine.The reaction temperature is not critical and can be convenientlyselected on the basis of the reaction medium chosen, the reactantconcentrations, and all other parameters useful for a successfulreaction exploitation. For example, the reaction can be conducted atroom temperature, but higher temperatures can also be used compatiblewith the reaction conditions.

[0024] In a second embodiment of the invention, the compound of formula(g) is the one in which X₁ is hydroxy, X₂ is methoxy, and Y is ahalogen, preferably chlorine. This preferred compound can be prepared,for example, starting from S-(−)-chlorosuccinic acid, as seen above, bytransformation via the corresponding anhydride. Different2-halogen-substituted succinic acids are prepared according to knownmethods.

[0025] The conversion is achieved by treating the S-(−)-chlorosuccinicacid with a dehydrating agent, preferably with acetyl chloride/aceticacid or with acetic anhydride, at a temperature ranging from roomtemperature to 90° C. Other modes of conversion, with other reactants,reaction media and conditions, which the expert technician can deducefrom his or her own general knowledge, are also possible. TheS-(−)-chlorosuccinic anhydride thus obtained is treated with a suitableamount of methanol to yield the compound of formula (U) desired.Compounds of formula (a) can be obtained, according to variants of thissecond embodiment of the invention, in which X₂ stands for one of themeanings envisaged, alkoxy or phenoxy, using suitable alcohol or phenolin the treatment of the starting anhydride.

[0026] In this second embodiment, the reducing agent is a mixed hydridesuch as lithium borohydride or lithium and aluminium hydride.

[0027] Carnitine inner salt is in turn obtained directly from thereduction product of 1-methyl hydrogen (S)-2-chlorosuccinate without theisolation of any intermediate product, with aqueous sodium hydroxide andtrimethylanmine, in the same way as described for the first embodiment.

[0028] In a third embodiment of the invention, the compound of formula(I) is the one in which X₁ and X₂ are a halogen, preferably chlorine,and Y is a halogen, preferably chlorine, and, more preferably X₁ and X₂and Y are chlorine. S-(−)-chlorosuccinic acid dichloride can be preparedstarting from S-(−)-chlorosuccinic acid with known reactions forobtaining acyl chlorides. The other halogen derivatives envisaged in theinvention can also be prepared in the same way.

[0029] In this third embodiment, the preferred reducing agent is sodiumborohydride.

[0030] Carnitine inner salt in turn is obtained directly from thereduction product of the previous reaction in exactly the same way as inthe cases described above.

[0031] In a fourth embodiment of the invention, the compound of formula(I) is the one in which X₁ and X₂ are hydroxy, and Y is the mesyloxygroup. Said compound can be prepared starting from S-malic acid andmethanesulphonyl-chloride with known hydroxy acid functionalisationreactions. The compound of formula (U) in which Y is tosyloxy isprepared in the same way.

[0032] In this fourth embodiment, the reducing agent is diborane.Carnitine inner salt is then obtained from the reduction product of theprevious reaction in exactly the same way as in the cases describedabove.

[0033] In a fifth embodiment of the invention, the compound of formula(I) is the one in which X₁ and X₂ are methoxy and Y is a halogen,preferably chlorine. Said preferred compound can be prepared asdescribed, for example, in J. Am. Chem. Soc. (1952), 74, 3852-3856,starting from S-(−)-chlorosuccinic acid and diazomethane or withmethanol and acid catalysis, preferably in the presence of dehydratingagents.

[0034] In this fifth embodiment, the preferred reducing agent is a mixedhydride such as lithium borohydride or lithium and aluminium hydride.

[0035] Carnitine inner salt is then obtained from the reduction productof the previous reaction in exactly the same way as in the casesdescribed above.

[0036] In a sixth embodiment of the invention, the compound of formula(I) is the one in which X₁ and X₂ are taken together and are an oxygenatom, and Y is a halogen, or a mesyloxy or a tosyloxy, preferably ahalogen. preferably chlorine.

[0037] This sixth embodiment shall be described in the foregoing in aparticularly detailed manner, being the preferred embodiment,comprising.

[0038] the transformation of S-(−)-chlorosuccinic acid into L-carnitinevia S-(−)-chlorosuccinic anhydride.

[0039] According to this embodiment, the process for the preparation ofL-carnitine inner salt includes the following steps:

[0040] a) transformation of S-(−)-chlorosuccinic acid into thecorresponding S-(−)-chlorosuccinic anhydride;

[0041] b) reduction of S-(−)-chlorosuccinic anhydride with a mixedhydride, in the presence of a solvent, obtaining a compound which,without being isolated, is directly converted to L-carnitine inner saltby treatment with an alkaline hydroxide and trimethylamine.

[0042] The reaction diagram illustrating this process is the following:

BEST METHOD OF CARRYING OUT THE PROCESS ACCORDING TO THE INVENTION

[0043] Preparation of S-(−)-chlorosuccinic Acid

[0044] One of the main problems posed in synthesis processes at theindustrial level is the ratio of the costs of reactants and materials,such as solvents and auxiliary substances, to the yield of the endproduct.

[0045] In industrial chemistry, increasingly frequent use is being madeof chiral compounds, the procuring of which on the market in substantialamounts is adversely affected by the high costs and difficulties inpreparation.

[0046] S-(−)-chlorosuccinic acid is still by no means easy to procure onthe market, suggesting that it may be economically convenient to prepareit within the context of one's own synthesis processes where it is usedas an intermediate.

[0047] In the reference cited above (J. A. Frick et al., 1992), thepreparation of S-(−)-chlorosuccinic acid is simply described as:“S-aspartic acid was converted to S-chlorosuccinic acid by treatmentwith sodium nitrite in hydrochloric acid”. In the diagram on page 621 ofthe reference cited, the yield of the synthesis step from S-asparticacid to S-chlorosuccinic acid is 70%. In the experimental part, the onlyexample of preparation is provided for S-bromosuccinic acid, with ayield of 88%. The bromosuccinic acid preparation conditions are not thesame as those described for chlorosuccinic acid, though they are takenas an example by analogy. From the industrial point of view, thesynthesis of bromosuccinic acid described by Frick et al. is not veryconvenient in economic terms. In the first place, the dilutions of thereaction mixture are very high; by way of an example, S-((+))-asparticacid is present to the extent of 5% w/v in the final reaction mixture. Adistinct disadvantage arises in the case of isolation of the end productby extraction, which requires a substantial amount of ethyl acetate inorder to obtain a 3% w/v solution of S-(−)-bromosuccinic acid. Inaddition, the end product, obtained with a stoichiometric yield of 88%,is not very pure, particularly as regards optical purity (e.e. =94%).

[0048] An improved process has now been found, in the course ofdevelopment of the invention described herein, for the preparation ofS-(−)-chlorosuccinic acid, which makes it possible to achieve an atleast 80%, approximately, higher yield, better conditions of reaction,especially in terms of reaction volumes, and of product isolation, andre-use of reactants, with consequent savings in terms of industrialprocess costs.

[0049] Therefore, the framework of the invention described herein coversa process for the preparation of S-(−)-chlorosuccinic acid whichincludes the reaction between S-((+))-aspartic acid and sodium nitritein a hydrochloric acid-aqueous milieu, in the presence of sodiumchloride, the improvement wherein consists in the isolation of thereaction product by precipitation by means of cooling of the reactionmixture.

[0050] Another object of the invention described herein is a process forthe preparation of S-(−)-chlorosuccinic acid which includes the reactionbetween S-((+))-aspartic acid and sodium nitrite in a hydrochloricacid-aqueous milieu, the improvement wherein consists in the use, as areaction medium, of the mother waters of a previous preparation reactionas in the process described above, said mother waters being used as atleast partial substitutes for the sodium chloride and hydrochloric acidenvisaged in the first process. According to this second process, thewashing waters of the end product of the previous reaction are also usedin addition to the mother waters.

[0051] The process for the preparation of S-(−)-chlorosuccinic acidaccording to the invention involves the reaction of S-((+))-asparticacid-with sodium nitrite, in the presence of sodium chloride andconcentrated hydrochloric acid.

[0052] The molar ratio of S-((+))-aspartic acid to sodium chlorideranges from 1:0.3 to 1:0.5, preferably from 1:0.35 to 1:0.45. Theprecipitation is done at a temperature ranging from −10° C. to −20° C.,and preferably at −15° C.

[0053] According to the invention described herein, the concentration ofS-((+))-aspartic acid is greater than 15%, and preferably 16% w/v in thereaction mixture.

[0054] In a first embodiment of this process, S-((+))-aspartic acid issuspended in demineralised water in a w/v ratio ranging from 1 kg/L to0.5 kg/L, preferably 0.66 kg/L, in the presence of sodium chloride, in amolar ratio as described above, and concentrated hydrochloric acid isadded in a ratio of S-((+))-aspartic acid to hydrochloric acid rangingfrom 0.35 kg/L to 0.55 kg/L, preferably 0.45 kg/L. The temperature ofthe mixture is brought down below 0° C., preferably to −5° C. In apreferred embodiment of the process, the reaction mixture is protectedin an inert atmosphere, e.g. nitrogen or argon. Sodium nitrite is thenadded in portions under stirring in a molar ratio ranging from 1.2 to2.5, preferably 1.78. The sodium nitrite can be added in solid form ordissolved in a suitable amount of water. When the sodium nitrite isadded in the form of a solution, the latter is suitably prepared usingpart of the water initially envisaged for the aspartic acid suspension.Addition of the sodium nitrite is made by monitoring the reactiontemperature.

[0055] The reaction progress can be monitored by observing thedevelopment of nitrogen. Once the reaction has begun, the development ofnitrogen may substitute for the inert atmosphere mentioned above.

[0056] To facilitate completion of the reaction, when the addition ofsodium nitrite is completed, the reaction temperature can also beraised, either by leaving it to rise spontaneously or by heating themixture. Preferably, the temperature should be brought up to 0° C.

[0057] The isolation of the product is done using conventional methods,but in the context of the invention described herein, it has been foundthat precipitation of the end product by cooling, e.g. to −15° C., isparticularly advantageous, especially as regards the presence of organicimpurities in the reaction mixture (unreacted fumaric, malic andaspartic acids).

[0058] From the industrial point of view, the process according to theinvention is advantageous whether applied in successive cycles orcontinuously. In fact, successive preparations of S-(−)-chlorosuccinicacid allow part of the reactants to be recovered.

[0059] The mother waters and possibly also the washing waters of thefirst reaction (REACTION A) are used as a reaction medium for asubsequent preparation (REACTION B). Advantageously, the mother watersof REACTION A (in actual fact, a brine) at low temperature (e.g. −15°C.) can be mixed with the other components of the following reaction(REACTION B), which leads to a frigorie saving and to a speeding-up ofprocess times. Advantageously, in REACTION A, the reaction medium alsoincludes the washing waters of REACTION B. In REACTION A, the reactionmedium can also include the washing waters of REACTION B.

[0060] Thus, another object of the invention described herein is aprocess for the preparation of S-(−)-chlorosuccinic acid which comprisesthe reaction between S-((+))-aspartic acid and sodium nitrite in ahydrochloric acid-aqueous milieu, the improvement wherein consists inthe use, as a reaction medium, of the mother waters of a previouspreparation reaction as described above, said mother waters being usedas at least partial substitutes for the sodium chloride and hydrochloricacid envisaged in the first reaction. Preferably, said mother waters areimmediately recycled at the S-(−)-chlorosuccinic acid precipitationtemperature envisaged for the previous reaction, so that, by mixing themwith the reactants still to be added, a temperature for the startingmixture around −5° C., being this latter the normal temperature for thisreaction, is immediately available. Advantageously, the washing watersfrom the previous reaction can also be used in addition to the motherwaters.

[0061] Alternatively, the process according to the invention includesthe reaction between S-((+))-aspartic acid and sodium nitrite in ahydrochloric acid-aqueous milieu, the improvement wherein consists inthe use, as a reaction medium, of the mother waters of a previouspreparation reaction, said mother waters being used as at least partialsubstitutes for the sodium chloride and hydrochloric acid envisaged andsaid S-(−)-chlorosuccinic acid is isolated by extraction. In this case,a yield of over 90% is obtained, without any inorganic residue.

[0062] Preferably, said mother waters are immediately recycled at theS-(−)-chlorosuccinic acid precipitation temperature envisaged for theprevious reaction, so that, by mixing them with the reactants still tobe added, a temperature for the starting mixture around −5° C., beingthis latter the normal temperature for this reaction, is immediatelyavailable. The process according to the invention is even moreadvantageous if inserted in the context of the process for thepreparation of L-carnitine, which is the object of the inventiondescribed herein.

[0063] In fact, the S-(−)-chlorosuccinic acid, obtained according to theprecipitation method described herein contains a percentage of sodiumchloride ranging from 15 to 25%, but can be used directly for thepreparation of S-(−)-chlorosuccinic anhydride, where the sodium chloridecontent can be easily eliminated.

[0064] Therefore, a further object of the present invention is a processfor the preparation of S-(−)-chlorosuccinic anhydride which includes thereaction between S-(−)-chlorosuccinic acid and acetic anhydride, theimprovement wherein consists in the use of crude S-(−)-chlorosuccinicacid coming directly from the processes described above.

[0065] Preparation of S-(−)-chlorosuccinic Anhydride

[0066] S-(−)-chlorosuccinic anhydride, which is obtained fromS-(−)-chlorosuccinic acid by converting the bicarboxylic acid into ananhydride, is a new compound and therefore the invention describedherein includes said compound as a reaction intermediate in the processdescribed here. The conversion occurs by treating S-(−)-chlorosuccinicacid with a dehydrating agent, preferably acetyl chloride/acetic acid oracetic anhydride, at a temperature ranging from room temperature to 90°C.

[0067] The carnitine inner salt is in turn obtained fromS-(−)-chlorosuccinic anhydride by reduction with a mixed hydride,preferably NaBH₄, in a suitable reaction medium, such as an organicsolvent, preferably aprotic, for example, tetrahydrofuran (THF),monoglyme, diglyme, dioxane, ethyl or methyl acetate (EtOAc or MeOAc) ora mixture of the same, and by reaction of the crude product thusobtained with aqueous sodium hydroxide and trimethylamine attemperatures ranging from room temperature to 120° C., preferably from60° C. to 100° C.

[0068] The compounds, 1-methyl hydrogen (S)-2-chlorosuccinate,(S)-2-chlorosuccinoyl dichloride and (S)-methanesulphonyloxysuccinicacid are new and are claimed herein as intermediates for the processaccording to the invention.

[0069] The L-carnitine inner salt can be salified with an acid, asindicated schematically here below:

[0070] where X-^(⊖) is, for example, a halide ion (preferably chloride),an acid sulphate, a methane sulphonate or an acid fumarate,

[0071]  or

[0072]  where X^(2- ⊖) is the counter-ion of a bicarboxylic acid, suchas, for example, a tartrate ion or a mucate ion.

[0073] Of course, all possible salifications with suitable counter-ionsare possible, normally counter-ions of non-toxic acids, accepted forpharmaceutical, alimentary and livestock breeding uses, and for the usesenvisaged for L-carnitine and its derivatives, e.g. the acyl carnitines,carnitine esters and acyl carnitine esters.

[0074] The following examples further illustrate the invention describedherein.

EXAMPLE 1

[0075] S-(−)-chlorosuccinic Acid “REACTION A”

[0076] To a vigorously stirred mixture of 200 g (1.50 mol) of L-asparticacid, 40 g of sodium chloride (0.68 mol), 440 ml (523.6 g) of 37% HCl(193.74 g of HCl, 5.32 mol), 200 ml of demineralised water, 100 ml ofwashing waters of the solid obtained in “REACTION B” (see Example 2),are added 184 g (2.66 mol) of solid sodium nitrite in approximately 2hours at a temperature of −5° C. under nitrogen blanket. Stirring iscontinued at the same temperature for 2.5 hours, the temperature israised to +0° C. in the space of approximately 1 hour, the mixture isleft at this temperature for another period of 1 hour and then thetemperature is lowered to −15° C. After 1.5 hours at that temperature,the mixture is vacuum filtered on Buchner filters and left to “drain”under vacuum pump aspiration for approximately 0.5 hours. The solid isthen washed with 80 ml of water at 0° C. and left on a vacuum filter foranother 1.5 hours.

[0077] The crude product is vacuum dried in an oven at 40° C. Itpresents approximately 15-20% sodium chloride contamination.

[0078] The molar percentages of the impurities present, calculated onthe basis of the NMR spectrum, are the following: fumaric acid 0.1-0.2%w/w malic acid 0.1-0.4% w/w aspartic acid 0.1-0.2% w/w

[0079] The yield of S-(−)-chlorosuccinic acid, calculated 100% pure, is80-81%.

EXAMPLE 2

[0080] S-(−)-chlorosuccinic Acid “REACTION B”

[0081] To a vigorously stirred mixture of mother waters and washingwaters (approximately 650 ml) from the previous reaction are added 200 g(1.50 mol) of L-aspartic acid, 360 ml (428.4 g) of 37% HCl (158.51 g ofHCl, 4.35 mol) and 100 ml of demineralised water; 184 g (2.66 mol) ofsolid sodium nitrite are then added in approximately 2 hours at atemperature of −5° C. under nitrogen blanket. Stirring is continued atthe same temperature for 2.5 hours, the temperature is raised to +0° C.in the space of approximately 1 hour, the mixture is left at thistemperature for another period of 1 hour, and the temperature thenlowered to −15° C. After 1.5 hours at this temperature, the mixture isvacuum filtered on Buchner filters and left to “drain” under vacuum pumpaspiration for approximately 0.5 hours. The solid is then washed with 80ml of water at 0° C. and left on a vacuum filter for another 1.5 hours.

[0082] The crude product is vacuum dried in an oven at 40° C. Itpresents approximately 15-20% sodium chloride contamination.

[0083] The molar percentages of the impurities present, calculated onthe basis of the NMR spectrum, are the following: fumaric acid 0.1-0.2%w/w malic acid 0.1-0.4% w/w aspartic acid 0.1-0.2% w/w

[0084] The yield of S-(−)-chlorosuccinic acid, calculated 100% pure, is86-87%.

[0085] The pure product, obtained by means of a further crystallisationof a sample of the crude product with water, has a melting point of180-182° C.

[0086] The overall yield of reactions A(+)B is 83-84%.

EXAMPLE 3

[0087] S-(−)-chlorosuccinic Anhydride

[0088] A suspension of 300 g (1.97 mol) of S-(−)-chlorosuccinic acid,containing 45-80 g of sodium chloride as a residue of the previouspreparation and 241.5 mL (2.56 mol) of acetic anhydride is stirred at52-55° C. for 3.5 hours. The insoluble sodium chloride is filtered outand the clear, colourless solution is vacuum evaporated and dried. Toeliminate the last residues of acetic acid and acetic anhydride, thesolid residue is extracted with 300 ml of anhydrous isopropyl ether, thesuspension is stirred vigorously for 5 minutes and filtered, and thesolid is washed on the filter with another 90 ml (66 g) of freshisopropyl ether. After vacuum drying in an anhydrous milieu, 251.3 g ofS-(−)-chlorosuccinic anhydride are obtained (95%; m.p. 75-80° C.;[α]D=−4.16 (c=1.0; ethyl acetate)).

EXAMPLE 4

[0089] S-(−)-chlorosuccinic Anhydride

[0090] A suspension of 53 g (0.347 mol) of S-(−)-chlorosuccinic acid in38 mL (0.40 mol) of acetic anhydride was stirred at 70° C. until thesolid was completely dissolved, after which the acetic acid and excessacetic anhydride were vacuum distilled. At this pointS-(−)-chlorosuccinic anhydride could be recovered by filtration, aftertreatment with cyclohexane, or by distillation at 0.5 mm Hg. Yields ofaround 95% were obtained in all cases (=44.4 g) (ee≧99%). ElementalAnalysis for: C₄ H₃ Cl O₃ C % H % Cl % Calc. 35.72 2.25 26.36 Found35.62 2.20 26.21

[0091]¹H NMR (CDCl₃, δ, p.p.m.): 3.21 (dd, J=18.7), 5.2, (1H, CHH—CO);3.59 (dd, J=18.7), 9.0, (1H, CHH—CO); 4.86 (dd, J=9.0, 5.2, 1H, CH—Cl);

EXAMPLE 5

[0092] 1-Methylhydrogen (S)-2-chlorosuccinate

[0093] To a solution of 6.00 g (0.0446 mol) of (S)-chlorosuccinicanhydride in 60 mL of CHCl₃, without ethanol, held at −65 C, was addedslowly a mixture of 1.80 mL (0.0446 mol) of MeOH in 20 mL of CHCl₃. Thesolution was maintained at the same temperature for 1 hour and then leftto rise to room temperature in 3 hours. After another 2 hours, thesolution was washed with 10 mL of NaOH 1N, dried on anhydrous sodiumsulphate and vacuum evaporated to dryness. After purification on achromatographic column, 5.94 g (80%) of the title compound wereobtained. H—NMR in DMSO-d₆: δ 2.89 (1H, dd, CHHCHCl), 3.00 (1H, dd,CHHCHCl), 3.71 (3H, s, COOCH₃), 4.78 (1H, t, CHCl).

EXAMPLE 6

[0094] (S)-2-chlorosuccinoyldichloride

[0095] A suspension of 10.00 g (0.0656 mol) of (S)-chlorosuccinic acidin 20.0 mL (0.274 mol) of thionyl chloride was refluxed for 1 hour.After cooling, the solution was vacuum evaporated to dryness. Theresidue was distilled at 90-93C/10 mm Hg to obtain 12.56 g (85%) of thetitle compound. H—NMR in DMSO-d₆: δ 3.50 (1H, dd, CHHCHCl), 3.60 (1H,dd, CHHCHCl), 5.20 (1H, t, CHCl).

EXAMPLE 7

[0096] (S)-methane-sulphonyloxysuccinic Acid

[0097] A solution of 8.04 g (0.060 mol) of (S)-malic acid and 9.2 mL(0.120 mol) of methanesulphonyl chloride in 60.0 mL of THF was refluxedfor 10 hours. After cooling, the solution was vacuum evaporated todryness to obtain 12.60 g (99%) of the title compound. H—NMR in DMSO-d₆:δ 2.41 (3H, s, CH₃SO₃), 2.90 (2H, m, CH₂), 5.47 (1H, t, CHOSO₂).

EXAMPLE 8

[0098] Dimethyl (S)-chlorosuccinate

[0099] To a solution of 7.04 g (0.046 mol) of (S)-chlorosuccinic acid in60 mL of methanol were added 2.0 mL of concentrated H₂SO₄. After 3 daysat room temperature the solution was vacuum evaporated and the residueextracted with EtOAc. The solution was washed with a 5% NaHCO₃ aqueoussolution and the organic phase was dried on Na₂SO₄. 7.90 g (94%) of thetitle compound were obtained by evaporation. H—NMR in DMSO-d₆: δ 3.00(1H, dd, CHHCHCl), 3.12 (1H, dd, CHHCHCl), 3.61 (3H, s, COOCH₃), 3.71(3H, s, COOCH₃), 4.77 (1H, t, CHCl).

EXAMPLE 9

[0100] L-carnitine Inner Salt by Reduction of (S)-2-chlorosuccinic Acid

[0101] To a suspension of 6.00 g (0.039 mol) of (S)-chlorosuccinic acidin 20 mL of anhydrous THF maintained at −15 C under nitrogen were added58.5 mL (0.0585 mol) of a 1M solution of borane in THF in 2 hours. After20 hours at the same temperature, the mixture was treated with 5.5 mL ofwater and left to stir at room temperature for 3 hours. After theaddition of 11 mL of 6M NaOH, the phases were separated. To the aqueousphase were added 7 mL of 40% Me₃N in water and the solution was left tostir at room temperature for 3 hours. The solution was vacuumconcentrated and the resulting solution brought to pH 5 with 37% HCl. Bymeans of evaporation of this solution a solid was obtained which wasextracted with 30 ml of MeOH. The solution obtained by filtration of theinsoluble part was vacuum evaporated and dried. The crude product waspurified on an ion-exchange column (Amberlite IR 120 form H⁺) by elutionwith 2% NH₄OH. By means of evaporation of the fractions containing thepure product, 3.14 g (50%) of L-carnitine were obtained.

EXAMPLE 10

[0102] L-carnitine Inner Salt by Reduction of 1-methyl hydrogen(S)-2-chlorosuccinate

[0103] To a suspension of 6.50 g (0.039 mol) of methyl(S)-2-chlorosuccinate in 30 mL of anhydrous DME held at −15C undernitrogen were added 0.87 g (0.040 mol) of 95% LiBH₄ in portions in 2hours. After 20 hours at the same temperature the mixture was treated asdescribed in Example 9 above to obtain 3.45 g (55%) of L-carnitine.

EXAMPLE 11

[0104] L-carnitine Inner Salt by Reduction of (S)-2-chlorosuccinoylDichloride

[0105] To a solution of 7.39 g (0.039 mol) of(S)-2-chlorosuccinoyl-dichloride in 30 mL of anhydrous DME held at −15Cunder nitrogen were added 0.74 g (0.0195 mol) of NaBH₄ in portions in 2hours. After 20 hours at the same temperature the mixture was treated asdescribed in example 9 to obtain 2.83 g (45%) of L-carnitine.

EXAMPLE 12

[0106] L-carnitine Inner Salt by Reduction of(S)-2-methanesulphonyloxysuccinic Acid

[0107] To a suspension of 8.27 g (0.039 mol) of(S)-methanesulphonyloxysuccinic acid in 30 mL of anhydrous THF held at−15C under nitrogen were added 58.5 mL (0.0585 mol) of a 1M solution ofborane in THF in 2 hours. After 20 hours at the same temperature, themixture was treated as described in Example 9 to obtain 2.51 g (40%) ofL-carnitine.

EXAMPLE 13

[0108] L-carnitine Inner Salt by Reduction of dimethyl(S)-2-chlorosuccinate

[0109] To a suspension of 7.04 g (0.039 mol) of dimethyl(S)-2-chlorosuccinate in 30 mL of anhydrous DME held at −15C undernitrogen, were added 0.69 g (0.030 mol) of 95% LiBH₄ in portions in 2hours. After 20 hours at the same temperature the mixture was treated asdescribed in Example 9 to obtain 3.32 g (53%) of L-carnitine.

EXAMPLE 14

[0110] S-(−)-chlorosuccinic Anhydride

[0111] A suspension of 53 g (0.347 mol) of S-(−)-chlorosuccinic acid in38 mL (0.40 mol) of acetic anhydride was stirred at 70° C. until thesolid had completely dissolved, after which the acetic acid and excessacetic anhydride were vacuum distilled. At this point, theS-(−)-chlorosuccinic anhydride could be recovered by filtration aftertreatment with cyclohexane, or by distillation at 0.5 mm Hg. Yields ofaround 95% (=44.4 g) were obtained in all cases. (ee≧99%). ElementalAnalysis for: C₄ H₃ Cl O₃ C % H % Cl % Calc. 35.72 2.25 26.36 Found35.62 2.20 26.21

[0112]¹H NMR (CDCl₃, δ, p.p.m.): 4.86 (dd, J=9.0 Hz, 5.2 Hz, 1H, CH—Cl);3.59 (dd, J=18.7 Hz, 9.0 Hz, 1H, CHH—CO); 3.21 (dd, J=18.7 Hz, 5.2 Hz,1H, CHH—CO)

[0113] L-carnitine Inner Salt

[0114] To a vigorously stirred suspension of 6.13 g (0.162 mol) of NaBH₄in 18 mL of anhydrous THF, held at 0° C., were added 43.4 g (0.323 mol)of S-(−)chlorosuccinic anhydride in 90 mL of anhydrous THF. Thesuspension/solution was stirred for 8 hours at that temperature, thenquenched with water, left to stir for one hour and then added with NaOH4N in two portions, the first to bring the suspension to pH 7.5 and thesecond, after vacuum evaporating the organic solvent, to ensure totaladdition of 0.484 mol of NaOH (in all, 121 mL). To said solution wereadded 51 mL (0.337 mol) of a 40% aqueous solution of Me₃N, and the wholewas transferred into a closed vessel and held for 16 hours at 70° C. Atthe end of the reaction, the residual trimethylamine was eliminated byvacuum evaporation and then 80.75 mL (0.323 mol) of HCl 4N were added.The solution, containing L-carnitine inner salt, together withapproximately 8% of impurities (mainly fumaric acid, maleic acid,hydroxycrotonic acid, D-carnitine) and sodium chloride, was-desalted byelectrodialysis and then vacuum dried. 38.5 g of a crude product wereobtained which was crystallised with isobutylic alcohol to yield 31.4 g(60.4%) of pure L-carnitine inner salt. (ee≧99.6%). Elemental Analysisfor: C₇ H₁₅ N O₃ C % H % N % Calc. 52.16 9.38 8.69 Found 52.00 9.44 8.59

[0115]¹H NMR (D₂O, δ, p.p.m.): 4.57 (m, 1H, CH—O); 3.41 (d, 2H,CH₂—COO); 3.24 (s, 9H, (CH₃)₃—N); 2.45 (d, 2H, CH₂—N)

[0116] L-carnitine Chloride

[0117] The reaction was repeated exactly as described above, exceptthat, at the end of the reaction in a closed vessel, the contents aftercooling were vacuum-dried. The residue was extracted with 53.5 ml (0.646mol) of 37% HCl and vacuum dried again. The residue was extracted twicewith ethanol; the first time with 200 mL and the second time with 60 mL,settling/filtering both times. The pooled ethanol solutions werevacuum-concentrated to a volume of approximately 50 mL, to which wereadded 600 mL of acetone to precipitate L-carnitine chloride. After onenight at room temperature the solid was filtered to yield 47.8 g ofcrude L-carnitine chloride. 38.5 g (60.4%) of pure L-carnitine chloridewere obtained by crystallisation with isopropanol (ee≧99.6%). ElementalAnalysis for C₇ H₁₆ Cl N O₃ C % H % Cl % N % Calc. 42.54 8.16 17.94 7.09Found 42.40 8.12 18.00 7.05

[0118]¹H NMR (CD₃OD, δ, p.p.m.): 4.58 (m, 1H, CH—O); 3.48 (m, 2H,CH₂—N); 3.27 (s, (CH₃)₃—N); 2.56 (d, J=6.7 Hz, 2H, CH₂—COOH)

1. A process for the preparation of L-carnitine inner salt comprisingthe reduction, with a hydride, of a compound of formula (I)

where: X₁ and X₂, which may be the same or different, are hydroxy, C₁-C₄alkoxy, phenoxy, halogen, or X₁ and X₂, when taken together are anoxygen atom and the resulting compound is a derivative of succinicanhydride; Y is halogen, the mesyloxy or the tosyloxy group: wherein thechoice of said hydride will be made in relation to the compound offormula (I) to be treated and subsequent treatment with a base and thenwith trimethylamine.
 2. A process according to claim 1, in which, in thecompound of formula (I), X₁ and X₂ are hydroxy and Y is chlorine, andthe reduction is done with diborane.
 3. A process according to claim 1,in which, the compound of formula (I), X₁ is hydroxy and X₂ methoxy, Yis halogen, and the reduction is done with lithium borohydride.
 4. Aprocess according to claim 1, in which, in the compound of formula (I),X₁ and X₂ are halogen, Y is halogen, and the reduction is done withsodium borohydride.
 5. A process according to claim 1, in which, in thecompound of formula (I), X₁ and X₂ are hydroxy, Y is the mesyloxy group,and the reduction is done with diborane.
 6. A process according to claim1, in which, in the compound of formula (I), X₁ and X₂ are methoxy, Y ishalogen, and the reduction is done with a mixed hydride.
 7. A processaccording to claim 6, in which the reduction is done with lithiumborohydride or with lithium and aluminium hydride.
 8. 1-methyl hydrogen(S)-2-chlorosuccinate as an intermediate in the process according toclaim 1 or
 3. 9. (S)-2-clorosuccinoyldichloride as an intermediate inthe process according to claim 1 or
 4. 10. Use of(S)-methanesulphonyloxysuccinic acid as an intermediate in the processaccording to claim 1 or
 5. 11. A process for the preparation ofL-carnitine inner salt according to the following reaction diagram:

comprising the following steps: transformation of S-(−)-chlorosuccinicacid into the corresponding S-(−)-chlorosuccinic anhydride; reduction ofS-(−)-chlorosuccinic anhydride with NaBH₄, in the presence of a solvent,obtaining a compound which, without being isolated, is directlyconverted to L-carnitine inner salt by treatment with water, then withan alkaline hydroxide and trimethylamine.
 12. A process according toclaim 11, in which the transformation in step a) occurs with adehydrating agent.
 13. A process according to claim 12, in which saiddehydrating agent is selected from the group consisting of acetylchloride/acetic acid and acetic anhydride, at a temperature ranging fromroom temperature to 90° C.
 14. A process according to claim 11, inwhich, in step b), the solvent is an aprotic organic solvent or amixture of organic solvents.
 15. A process according to claim 14, inwhich said aprotic solvent is selected from the group consisting oftetrahydrofuran, monoglyme, diglyme, dioxane, ethyl acetate. 16.S-(−)-chlorosuccinic anhydride as an intermediate in the process ofclaims 12-15.
 17. A process for the preparation of S-(−)-chlorosuccinicacid comprising the reaction between S-((+))-aspartic acid and sodiumnitrite in a hydrochloric acid-aqueous milieu being saidS-((+))-aspartic acid suspended in demineralised water in a w/v ratioranging from 1 kg/L to 0.5 kg/L, and concentrated hydrochloric acidbeing added in a ratio of S-((+))-aspartic acid to hydrochloric acidranging from 0.35 kg/L to 0.55 kg/L, in the presence of sodium chloride,said S-((+))-aspartic acid and said sodium chloride being in a molarratio ranging from 1:0.3 to 1:0.5, the improvement wherein consists inthe isolation by precipitation of the reaction product by cooling thereaction mixture at a temperature ranging from −10° C. to −20° C.
 18. Aprocess according to claim 17, in which said temperature is −15° C. 19.A process for the preparation of S-(−)-chlorosuccinic acid comprisingthe reaction between S-((+))-aspartic acid and sodium nitrite in ahydrochloric aid-aqueous milieu, the improvement wherein consists inusing as the reaction medium mother waters from a previous preparationreaction as described in claim 17, said mother waters being used as atleast partial substitutes for the sodium chloride and hydrochloric acidenvisaged in claim
 17. 20. A process according to claim 19, in whichsaid mother waters are used at the precipitation temperature ofS-(−)-chlorosuccinic acid envisaged in the process as described in claim17 or
 18. 21. A process according to claim 19 or 20, in which washingwaters are used in addition to mother waters.
 22. A process according toclaim 17, in which the reaction medium comprises washing waters from theprocess described in claim
 19. 23. A process for the preparation ofS-(−)-chlorosuccinic acid comprising the reaction betweenS-((+))-aspartic acid and sodium nitrite in a hydrochloric acid-aqueousmilieu, the improvement wherein consists in using as the reactionmedium, the mother waters of a previous preparation reaction asdescribed in claim 17, said mother waters being transferred to thereactor at the S-(−)-chlorosuccinic acid precipitation temperature andas at least partial substitutes for the sodium chloride and hydrochloricacid envisaged in claim 17, and said S-(−)-chlorosuccinic acid beingisolated by extraction.
 24. A process for the preparation ofS-(−)-chlorosuccinic anhydride which comprises the reaction betweenS-(−)-chlorosuccinic acid and acetic anhydride, the improvement whereinconsists in the use of crude S-(−)-chlorosuccinic acid coming directlyfrom the process described in any of claims 17-22.
 25. A processaccording to claim 17, in which, alternatively, S-(−)-chlorosuccinicacid comes directly from the process described in claim
 23. 26. Aprocess according to any of the foregoing claims, in which theL-carnitine inner salt is subsequently transformed into one of itssalts.
 27. A process according to claim 26, in which said salt is apharmaceutically acceptable salt.